There are substantial problems in sending quantum information safely over free-space channels because of air turbulence, diffraction, beam divergence, and noise. This study offers an extensive performance evaluation of quantum computing-based security methods utilised in free-space communication systems, specifically emphasising Quantum Secure Direct Communication (QSDC) and Quantum Key Distribution (QKD) protocols. The purpose of this work is to examine the attainable secret-key generation rates, assess the robustness of quantum security protocols in practical free-space environments, and contrast their effectiveness in preventing eavesdropping and signal degradation. This research employs quantum optics, turbulence theory, and quantum information theory to examine the performance constraints of continuous-variable (CV) coherent-state protocols amid fluctuating turbulence intensities. The suggested analytical approach simulates both fading and non-fading atmospheric channels to calculate essential rates and quantum bit error rates (QBER) for free-space quantum secure direct communication (QSDC) systems. Experimental findings and simulation outcomes indicate that free-space quantum channels can provide high-rate and long-distance secure communication when optimized using pilot-guided coherent-state protocols. The review shows how QSDC is better than regular QKD since it lets messages be sent directly without pre-shared keys. It also talks about how useful it may be for secure communication networks between satellites and between satellites and the earth. The paper closes by delineating the primary research deficiencies and suggesting avenues for improving protocol efficiency, error rectification, and turbulence mitigation in forthcoming global quantum communication systems.
There are substantial problems in sending quantum information safely over free-space channels because of air turbulence, diffraction, beam divergence, and noise. This study offers an extensive performance evaluation of quantum computing-based security methods utilised in free-space communication systems, specifically emphasising Quantum Secure Direct Communication (QSDC) and Quantum Key Distribution (QKD) protocols. The purpose of this work is to examine the attainable secret-key generation rates, assess the robustness of quantum security protocols in practical free-space environments, and contrast their effectiveness in preventing eavesdropping and signal degradation. This research employs quantum optics, turbulence theory, and quantum information theory to examine the performance constraints of continuous-variable (CV) coherent-state protocols amid fluctuating turbulence intensities. The suggested analytical approach simulates both fading and non-fading atmospheric channels to calculate essential rates and quantum bit error rates (QBER) for free-space quantum secure direct communication (QSDC) systems. Experimental findings and simulation outcomes indicate that free-space quantum channels can provide high-rate and long-distance secure communication when optimized using pilot-guided coherent-state protocols. The review shows how QSDC is better than regular QKD since it lets messages be sent directly without pre-shared keys. It also talks about how useful it may be for secure communication networks between satellites and between satellites and the earth. The paper closes by delineating the primary research deficiencies and suggesting avenues for improving protocol efficiency, error rectification, and turbulence mitigation in forthcoming global quantum communication systems.
The study of free-space quantum communications requires tools from quantum information theory, optics, and turbulence theory.Here we combine these tools to bound the ultimate rates for key and entanglement distribution through a free-space link, where the propagation of quantum systems is generally affected by diffraction, atmospheric extinction, turbulence, pointing errors, and background noise.Besides establishing ultimate limits, we also show that the composable secret-key rate achievable by a suitable (pilot-guided and postselected) coherent-state protocol is sufficiently close to these limits, therefore showing the suitability of free-space channels for high-rate quantum key distribution.Our paper provides analytical tools for assessing the composable finite-size security of coherent-state protocols in general conditions from the standard assumption of a stable communication channel (as is typical in fiber-based connections) to the more challenging scenario of a fading channel (as is typical in free-space links).
The study of free-space quantum communications requires tools from quantum information theory, optics and turbulence theory. Here we combine these tools to bound the ultimate rates for key and entanglement distribution through a free-space link, where the propagation of quantum systems is generally affected by diffraction, atmospheric extinction, turbulence, pointing errors, and background noise. Besides establishing ultimate limits, we also show that the composable secret-key rate achievable by a suitable (pilot-guided and post-selected) coherent-state protocol is sufficiently close to these limits, therefore showing the suitability of free-space channels for high-rate quantum key distribution. Our work provides analytical tools for assessing the composable finite-size security of coherent-state protocols in general conditions, from the standard assumption of a stable communication channel (as typical in fiber-based connections) to the more challenging scenario of a fading channel (as typical in free-space links).
This research explores the fundamental principles of quantum computing and their application in enhancing communication security through Quantum Key Distribution (QKD) in free-space channels. Classical cryptographic methods face inherent limitations, particularly with the advent of quantum computational capabilities. Quantum mechanics, specifically the principles of superposition and entanglement, offers a robust framework for secure communication, ensuring that any attempt at eavesdropping inevitably perturbs the quantum state, thus revealing the intruder's presence. Through comprehensive simulations, this study analyzes the performance of QKD systems in free-space communication. Key parameters such as transmittance efficiency ( ), Quantum Bit Error Rate (QBER), and Secret Key Rate (SKR) are evaluated. Results indicate that transmittance efficiency in free space generally decreases with increasing zenith angle, with downlink scenarios (day and night) exhibiting higher efficiencies compared to uplink during the night. For all protocols, simulations demonstrate an increase in QBER and a corresponding decrease in SKR as the zenith angle increases, with downlink performance consistently surpassing uplink in terms of a higher SKR and a lower QBER
Quantum Key Distribution (QKD) has emerged as a fundamentally secure approach to communication in the era of quantum computing, offering protection against threats posed to classical cryptographic schemes such as RSA and Diffie-Hellman. This thesis presents a comprehensive performance analysis of satellite-based QKD protocols, focusing on both prepare-and-measure and entanglement-based schemes under realistic atmospheric and operational conditions. The study begins by introducing the theoretical foundations of quantum communication, including qubits, entanglement, and quantum entropy, and motivates the need for satellite-based QKD to overcome the distance limitations of fiber-based systems. Subsequently, the thesis evaluates four prominent QKD protocols-BB84, B92, BBM92, and E91-using a circular beam propagation model that incorporates atmospheric effects such as diffraction, turbulence, attenuation, and pointing errors, along with environmental noise contributions for uplink and downlink. Comparative numerical simulations reveal that protocol performance is strongly influenced by channel asymmetries, beam propagation characteristics, and noise, providing guidance on optimal protocol selection for low Earth orbit (LEO) satellite links. The research further investigates high-dimensional (HD) QKD protocols, specifically HD-BB84 and HD-Extended B92, using the elliptic-beam approximation to account for turbulence-induced distortions for both uplink and downlink. Simulations under vary ing system dimensions, weather conditions, and zenith angles demonstrate that HD-BB84 achieves higher key rates, superior noise tolerance, and more favorable probability distributions of the key rate compared to HD-Extended B92, highlighting the advantages of high-dimensional encoding for robust satellite-based QKD.
Unmanned aerial vehicles (UAV) based on free-space optical (FSO) systems combine the benefits of both the high data rate of the FSO and the mobility of UAV. However, cumulative effects of laser beam divergence and turbulence-induced fading on the received irradiance in the FSO link might allow an external eavesdropper located near the authorized receiver to break the transmission under certain conditions. Quantum key distribution (QKD) is a technique of secure communication that employs quantum mechanics-based cryptography principles. A combination of a continuous-variable quantum key distribution (CV-QKD) and Gaussian modulation of quantum coherent states (GMCS) enables a relatively secure communication system against collective attacks over FSO transmission links. However, the performance is restricted and degraded due to the influence of atmospheric turbulence and excess noise. This paper aims to evaluate the performance of a secured UAV-based FSO system utilizing prepare and measure CV-QKD protocol based on GMCS under collective attack in terms of quantum bit error rate (QBER), outage probability, and secret key rate (SKR) considering the impairments of the FSO channel, link budget, deviations in the position and attitude angles of the UAV, channel fluctuating transmittance, and excess noise. The system parameters, including transmit power, receiver’s field-of-view, and beam divergence angle, are optimized to satisfy the system’s design criteria of threshold-QBER=10−3and threshold outage probability=10−2. The proposed system tolerates boresight displacement up to 7 cm with the optimized values. The Closed-form expressions and analytical results are confirmed by Monte-Carlo simulations.
Satellite-based free-space quantum key distribution (QKD) provides a practical framework for achieving secure global communication beyond the limitations of optical fibers. In this work, the quantum bit error rate (QBER) and secure key rate of four representative protocols-BB84, B92, BBM92, and E91 are investigated over low earth orbit (LEO) links in both uplink and downlink configurations. The optical link is modeled using a Gaussian beam formalism, incorporating the effects of diffraction, pointing errors, atmospheric turbulence, and background noise contributions. The protocols are examined under day and night-time operating conditions, and their dependence on the zenith angle is analyzed. The findings show that downlink links generally exhibit lower QBER and higher secure key rates than uplinks, and among prepare-and-measure schemes, BB84 consistently outperforms B92, while in entanglement-based approaches, BBM92 achieves higher key rates than E91.